This invention generally relates to a thin film semiconductor device and a method for fabricating it. More specifically, this invention relates to such a thin film semiconductor device as being suitably used in a display device in an active matrix (AMX) system and a method for fabricating it.
In recent years, polycrystalline silicon (referred to as poly-Si) has been used as a material for a thin film transistor (referred to as TFT), which is a thin film semiconductor device for a display device in an AMX system, since poly-Si provides high image quality.
Conventionally, the poly-Si has been made by means of a low pressure CVD (referred to as LPCVD) technique. As an insulating substrate in making the poly-Si, a quartz glass or a common glass plate is used. The common glass is subjected to a great limitation since it can only be used at a process temperature up to about 640.degree. C. There have been proposed several techniques for making poly-Si of high degree of crystallinity in such a low temperature process. A first example thereof is to increase the deposition temperature to the temperature (630.degree. C.) close to the possible highest process temperature, to provide a deposition pressure of 0.3 Torr, thereby decreasing the deposition speed of the LPCVD film so as to enhance the crystallinity of the deposited film (the total volume of the crystalline component contained in a unit volume) (For example, see Japan Display '86 Tech. Digest 3.5 (deposition temperature: 630.degree. C.)). The second example thereof is to deposit the LPCVD film at 600.degree. C. and enhance its crystallinity through the subsequent heat treatment at about 600.degree. C. (For example, Japan Society for the Promotion of Science 147-th Commitee 7-th Search Report (1985, 3. 19) page 24). The third example thereof is to deposit the LPCVD film at 610.degree. C., make it amorphous through ion implantation, and enhance its crystallinity through the subsequent heat treatment at 600.degree. C. (For example, see the theses for 33-rd Japan Society of Applied Physics (Spring 1986) page 544 (deposition temperature: 610.degree. C.)). The other examples are disclosed in NIKKEI ELECTRONICS 1984, 9. 10 page 211 (deposition temperature: 600.degree. C.) and Journal of Electrochemical Society, 127, 686 (1980), 131, 676 (1984), etc.
The resultant poly-Si film provided by the techniques mentioned above is a film of a dominant (110) texture. The above methods can improve the crystallinity to a certain degree but can not provide a sufficient carrier mobility when a TFT has been fabricated.
Further, the convention display device in AMX system imvolves several problems because of insufficiency of the carrier mobility. The first problem is that a long addressing time for a peripheral driver circuit which is formed on the same substrate as the display part, is spent. This problem sets a limit to increase of the number of pixels on the display part, and limits image quality. The second problem is that the size of TFT on the display part can not be greatly reduced so that the aperture rate will not be increased, thereby providing unsatisfactory image quality.